1. Field of the Invention
The present invention relates to a surface acoustic wave device which can be applied to a high frequency filter, for example, and more particularly, it relates to a surface acoustic wave device employing diamond layers.
2. Description of the Background Art
In a surface acoustic wave device, which can be miniaturized with a small number of elements, it is possible to input/output signals on a surface wave propagation path. Thus, the surface acoustic wave device is applied to a filter, a delay line, an oscillator, a resonator, a convolver, a correlator and the like. In particular, a surface acoustic wave filter has been practically applied to an intermediate frequency filter for a television for a long time, while the same is now being applied to filters for VTR and various communication devices.
A surface acoustic wave device, for example, has been generally manufactured by forming interdigital electrodes on a piezoelectric single-crystalline member of LiNbO.sub.3 or LiTaO.sub.3. Additionally, a piezoelectric thin film of ZnO or the like is recently formed on a substrate of glass or the like by a technique such as sputtering.
In relation to a surface acoustic wave filter, on the other hand, awaited is a device which can be employed at a higher frequency. In order to attain a higher center frequency in the frequency characteristics of a surface acoustic wave device, it is necessary to reduce electrode spacing of interdigital electrodes, or to increase the speed of surface waves. However, reduction in electrode spacing of the interdigital electrodes is restricted for limitation of the fine processing technology using lithography etc. To this end, there has been developed a surface acoustic wave device obtained by stacking a piezoelectric film on a material such as sapphire or diamond, for example, which propagates surface acoustic waves at a higher speed (refer to Japanese Patent Publication No. 54-38874 (1979) or 64-62911 (1989), for example).
In employment of such a surface acoustic wave device, characteristic changes of the device following temperature changes have recently come into question. Such temperature changes include that intrinsically caused by continuous application of an electric field to the surface acoustic wave device, and that extrinsically caused by an air temperature change and environmental variation. While these temperature changes cause characteristic changes such as changes of the center frequency, insertion loss, the frequency band being important for filter characteristics, and the like, the center frequency is extremely changed in particular. In a device, for example, which has a ZnO piezoelectric film formed on a diamond layer, the frequency is changed at a higher rate of 30 ppm/.degree.C. Therefore, when the surface acoustic wave device is applied to a narrow-band intermediate frequency filter which is employed in general, for example, the center frequency is extremely varied to result in instability and impracticality.